An EVOH is excellent in melt moldability, gas barrier properties, water resistance, oil resistance, antistatic property and mechanical strength, and are used as various types of packages in the form of a film, a sheet, a container or the like. In these packages, visible imperfections generated at the time of molding, e.g. fish eyes, are significant problems that need to be addressed. Thus, traditionally, to inhibit the above-mentioned visible imperfections, various improvements in the process of producing an EVOH have been proposed.
For example, JP 1(1989)-52404B discloses a method including a step of copolymerizing ethylene and vinyl acetate, in which the ratio of the average residence time within a polymerization vessel with respect to the half value period of a polymerization initiator is not less than a fixed coefficient, which is determined from the polymerization degree of vinyl acetate. This method can be realized by using a polymerization initiator having a short half-life.
Furthermore, for example, JP 9(1997)-71620A discloses a method in which a conjugated polyene compound is added after copolymerizing ethylene and vinyl acetate, and further unreacted materials are removed. Also, JP 10(1998)-158325A discloses a method in which a thiol compound is added after copolymerizing ethylene and vinyl acetate, and further unreacted materials are removed.